The present invention relates to a method of measuring the dopant concentration profile of a semiconductor and, more particularly, to a method of measuring the two-dimensional concentration profile of a dopant existing in a source/drain region.
Microconfigurations of semiconductor devices have given rise to various problems to be solved. Among them, a threshold voltage increasing with a decrease in gate length, i.e., a short channel effect is critical. While studies using simulation or similar implementation are under way in search of a solution to the short channel effect problem, the measurement of the two-dimensional concentration profile of a dopant in a source/drain region is essential for more reliable design of semiconductor devices.
It has been customary to measure the one-dimensional concentration profile of a dopant by secondary ion mass spectroscopy (SIMS) or spreading resistance (SR) measurement. Although this kind of method is applicable to the measurement of depth profiles, it cannot perform two-dimensional measurement. While a two-dimensional display mode, called mapping, has recently been added to SIMS, the probe diameter of primary ions used by SIMS is usually of the order of microns and cannot implement a resolution sufficient for the evaluation of a source/drain region. A decrease in the probe diameter of SIMS, if realized, would lower sensitivity due to the principle of measurement particular to SIMS. In practice, therefore, the two-dimensional dopant concentration profiles is not achievable with SIMS.
To measure a two-dimensional dopant concentration profile, there have been proposed methods using microscopy. For example, a measuring method using a scanning electron microscope (SEM) is disclosed by Venables et al in Proc. 3rd Intern. Workshop on the Measurements and Characterization of Ultra-Shallow Doping Profiles in Semiconductors, pp. 43-1-43.7, 1995. The SEM scheme, however, has a problem that the charge-up of an insulator and contrast ascribable to configuration lower the reliability of contrast derived from the discharge of secondary electrons which is dependent on the dopant concentration In light of this, a method using chemical etching has also been proposed. This method etches a semiconductor with an etchant of the kind etching a semiconductor by an amount dependent on the dopant concentration, and then measures the etched configuration so as to match it to dopant concentrations.
The etched configuration of the semiconductor may be measured by a transmission electron microscope (TEM), as taught by Spinella et al in Proc. 3rd Intern. Workshop on the Measurements and Characterization of Ultra-Shallow Doping Profiles in Semiconductors, pp. 42.1-42.8 by way of example. However, the problem with the TEM scheme is that a substantial period of time is necessary for sample preparation.
To solve the above problem, the etched configuration may be measured by a scanning probe microscope (SPM), as also proposed in the past. Among various SPMs, a scanning tunnel microscope disclosed in Japanese Patent Laid-Open Publication No. 4-111337 was used first. However, because this type of microscope measures a tunnel current to flow between a probe and the sample surface, it cannot measure insulators. Moreover, the scanning tunnel microscope causes the removal of a native oxide layer or protection layer and other surface treatment to effect the quality of data. In light of this, there has been proposed to replace the scanning tunnel microscope with an atomic force microscope (AFM) capable of measuring an attraction or a repulsion force between its probe and the sample surface. Today, the chemical etching followed AFM scheme is predominant over the others as to the measurement of a dopant concentration distribution of a semiconductor device usually including a patterned oxide film or similar insulation film. Raineri et al, for example, teach in Appl. Phys. Lett., Vol. 64, pp. 354-356, January, 1994 a method which etches a p-type diffusion region with a hydrofluoric acid, nitric acid and acetic acid mixture with a ratio of 1:3:8 under ultraviolet irradiation, and then measure the etched configuration which depends on dopant concentration by AFM, However, when the amount of etching increases, the concentration distribution does not match in the portion adjoining the surface.
Among various technologies available for fabricating a source/drain region, ion implantation which drives ions containing a dopant into a semiconductor by accelerating them is extensively used because of its desirable controllability and reproducibility. The concentration of a dopant driven by ion implantation has a peak in the vicinity of the surface of a semiconductor. On the other hand, a hydrofluoric acid and nitric acid mixture often used as an etchant for silicon is dependent on the dopant concentration such that the amount of etching sharply increases in a high concentration region. As a result, a two-dimensional dopant concentration distribution cannot be reliably measured in such a region.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 8-285519, IEEE ELECTRON DEVICE LETTERS, VOL. 16, NO. 3, MARCH 1995 by Barrette et al, and Nuclear Instruments and Methods in Physics Research B96 (1995), pp. 123-132 by Vandervorst et al.